Matched filters and mixers are widely exploited in communication systems and capacitive touch control systems. In general, a mixer may be realized by a multiplier, which generates a multiplication result of a received signal and a local signal. In addition, the mixer may be further realized by a switching mixer with high linearity and low noise. The switching mixer is equivalent to multiplying the received signal by a square wave (i.e., the local signal). However, either the square wave or the sinusoidal wave has sidelobe in frequency domain, and an extra noise is brought in, such that a system SNR (Signal to Noise Ratio) is lowered. In order to solve problem of noise brought by the sidelobe, window function may be applied before the integrator. As FIG. 1 shows, a signal SIG1 represents a waveform without applying any window function, and a signal SIG2 represents a waveform applying a window function e. As can be seen from FIG. 1, the window function e may be regarded as an envelop of the signal SIG2. Applying the window function e may effectively suppress noise brought by sidelobe, and enhance an anti-interference capability around the frequency band, such that the system SNR is enhanced.
The effect of window function may be realized by digital integrator, where the digital integrator may use different integrating gains at different time intervals, to achieve the effect of applying window function. However, an output frequency of the digital integrator is high, which is not suitable for the design of the front-end analog-to-digital converter (ADC). In other words, the front-end analog-to-digital converter needs to have sufficient high sampling rate to accurately perform sampling on the high frequency output signal, where the power consumption and complexity of the circuit are raised. Therefore, it is necessary to improve the related art.